If you google this directory you come to a thread at nexusmods where this error is described. I dont use mods (yet - i wait till final release) so i have no idea if this is true or not. But as it seems you dont need to have installed a "true" mod, the modloader BepinEx creates this.
In your valheim-folder there are old fragments of (previously used) mods/modloader left. When you delete these (or update BepinEx), it should run normally, maybe you have lost your world/character save because of that but maybe not, you will see.
There is a reason why everybody warns using mods on an game in development.
You did it on your own risk!

https://www.google.com/search?q=%22common%2FValheim%2Fvalheim_Data%2FMono%2F%22

Edit: See #4
https://steamcommunity.com/app/892970/discussions/1/3154202776531668586/

More Infos where you find all files:
https://steamcommunity.com/app/892970/discussions/1/3409804177175439326/